By way of background, ultrasound transducers in medical ultrasound emitting devices can be used for purposes such as therapeutic, imaging, or interventional ultrasound. The total power emitted by the transducer head is an important parameter that needs to be measured and calibrated prior to use on patients. In addition, the power emissions should be monitored periodically during normal operation and/or when servicing/repairing the ultrasound equipment or device. These ultrasound emitting devices are also regulated by certain industry standards for therapeutic, imaging, and interventional ultrasound.
Current apparatuses and methods of measuring ultrasound power from these devices can be time consuming (involving the time and high cost of skilled technicians), bulky (with limited or only partial portability), or not sufficiently accurate. Inaccuracy is particularly a problem when the ultrasound emitting head on the device to be measured has an array of transducers (either multiple individual transducers, or multiple transducer elements on a single transducer substrate) and/or complex shape or beam profiles. In some cases, the ultrasound can have diverging or converging beams, or both diverging and converging beams but in different planes, making the ultrasound power difficult to measure.
Currently the industry approach to determine the total emitted power of an ultrasound head is to use either a scanning technique with a calibrated hydrophone, a reactive force balance technique, or a thermoacoustic technique. Please see FIG. 1 for examples of these prior art devices. These existing apparatuses and techniques have certain limitations and disadvantages.
The hydrophone technique is very time consuming as it requires an accurate measurement of the ultrasound wave pressure in multiple points. Since both the ultrasonic beam from the transducer, and the receiving response of the hydrophone are strongly directional, alignment is crucial in the measurement procedure. In order to align the hydrophone properly, a suitable mount must be used. There are two types of hydrophones: needle and membrane. Membrane hydrophones should be clamped to the mount by their supporting ring with the front of the hydrophone (the side where the ring is almost flush with the membrane) nearest the transducer. Probes/needles should be clamped several centimeters back from the sensitive element. The hydrophone mount should have five degrees of freedom. It should be possible to move the hydrophone by translation in three orthogonal directions, one of which should be parallel to the direction of propagation of the ultrasound. It should also be possible to rotate and tilt the hydrophone, ideally with the axes of rotation and tilt passing through the active element.
The radiative force balance technique can provide accurate measurements for the transducer heads that emit ultrasound wave with low divergence (plane waves/parallel beam), however it can only measure power from a single transducer sitting flat and aligned on top of the cone target.
These balance instruments use a positioning clamp to hold the transducer in de-gassed water above a conical target. The ultrasonic energy passes through the water, reflects from the target cone, and is then absorbed by the rubber lining. The radiant power is directly proportional to the total downward force on the target. This force is then transferred through the target support assembly to a digital scale that displays the power in watts of power or grams of force. These power meters however, cannot measure power from a non-planar transducer, non-planar arrays of transducers, or transducers emitting inside a cavity. There remains a need to provide apparatuses and methods for measuring and characterizing ultrasound, that can overcome the shortcomings of the prior art.